The present invention relates to hoisting machines such as rope hoists or chain blocks and, more particularly, to such hoisting machines provided with an overload protection apparatus.
In general, for heavy duty hoisting machines, a maximum allowable weight that can be handled without danger is given as a rated load. However, since the operator does not always know the exact weight of the cargoes to be handled, weights exceeding the rated load are often applied to the hoisting machines, resulting in an overload operation. In order to avoid the danger of the overload operation, it is necessary to provide the hoisting machines with a suitable overload protection apparatus.
Hitherto, a variety of overload preventing apparatus have been proposed. One of them incorporates a mechanical balance adapted to produce a warning signal or to disconnect the hoist from the driving motor. In another overload protection apparatus, the load is sensed as the current through the driving motor during the hoisting. Thus, when an excessively large current is sensed, a warning is produced and the motor is stopped. Still another overload protection apparatus incorporates a mechanical torque limiter such as slip gears which slips for a torque exceeding a predetermined torque threby to stop the hoisting.
The first one has been found inconvenient in that it necessitates a number of parts and expensive equipments for an accurate weighing. In addition, this type of overload protection apparatus requires a troublesome step of hoisting up the load for weighing the load.
The second type of overload protection apparatus has been found also inconvenient in that it requires complicated controlling circuits, as well as troublesome works of adjustment and usual maintenance, although it can provide a considerably accurate overload protection operation.
The third type of overload protection apparatus can perform the required function relatively easily with a highly simplified structure. However, there is a fear that the load once hoisted may be lowered unitentionally. The third type of overload protection apparatus would be the most acceptable one, if it is free from the danger of the unitentional and uncontrolled lowering.
When one gear or wheel constituting the reduction gears is designed to rely upon a frictional force, such that the wheel can transmit a torque smaller than a predetermined one but slips for a larger torque, the reduction gears themselves act as the overload protection apparatus which prevents the hoisting of an excessive load. However, a problem takes place when a load which is slightly smaller than the load causing the slipping of the wheel is hoisted. Namely, the hoisting machines are subjected to a larger torque when they are operated to adjust the height or position of the hoisted load by so called "inching operation" than when the load is initially lifted from the ground. Thus, when the "inching operation" is made for a load which is scarecely born by the slipping wheel, the resultant increased torque would exceed the capacity of the slipping wheel to allow the later to slip resulting in the unintentional and uncontrolled lowering of the hoisted load.
Once the uncontrolled lowering takes place, the frictional resistance exerted by the slipping wheel is changed to a kinetic frictional force which is, of course, smaller than the static frictional force by which the load has been hoisted, so that the lowering of the load can no more be stopped.
This means that the overload protection apparatus which is designed as a safety device may undesirably incur a danger.